This invention relates to a predistorter arrangement for linearising an amplifier. In particular, the present invention relates to a polynomial predistorter arrangement designed to compensate for IMD distortion in an amplifier using multiple order polynomial approximation techniques.
In an ideal system, a linear amplifier provides uniform gain throughout its dynamic range in order that the output signal of the amplifier is a correct, amplified version of the input signal. In reality however all linear amplifiers exhibit non-ideal properties such as amplitude and phase distortion, which are undesirable and can seriously deteriorate the performance of a system. One effect of this non-linearity of the amplifier is the generation of output frequencies equal to the sums and differences of integer multiples of the input frequency components. This effect is known as intermodulation distortion (IMD) and is particularly undesirable in high-power radio frequency (RF) amplifiers designed for use in broadband systems. For example, a broadband amplifier used in the TDMA cellular system will generate various intermodulation products as a result of amplifying a multitude of TDMA channels occurring at fixed frequency intervals across a TDMA band, with coincident active frames.
A number of linearisation techniques have been developed to overcome the above distortion problems associated with a linear amplifier. A few of these techniques operate in real-time to account for time dependent changes in the non-linear characteristics of the amplifier. Such changes may result from, for example, temperature variations in the amplifier, ageing of amplifier components, power supply fluctuations, or, most particularly, changes in the operating point of the amplifier due to a change in the number or power of the input carriers. Of the broadband, RF-based linearisation techniques, the two most commonly used are feed forward linearisation and predistorter linearisation.
A feed forward linearisation mechanism relies on creating an error signal representative of the IMD products introduced by the linear amplifier, and feeding this signal forward to combine with the output spectrum of the amplifier, cancelling out the unwanted distortion. In order for the cancellation process to operate correctly, it is necessary for the mechanism to accurately adjust the amplitude and phase of the error signal prior to combining it with the output of the amplifier. This typically involves the use of additional amplifiers and lossy delay lines and couplers appearing in the output path from the main amplifier. These losses and the requirement for additional amplifiers, which are not adding to the output power of the system, result in a low-efficiency solution.
In general, predistortion linearisation mechanisms involve deliberate alteration of the relatively low level input signal to the amplifier in anticipation of the undesired distortion process occurring within the amplifier. Specifically, the mechanism predistorts the input signal in an inverse sense to the distortion produced by the amplifier such that in series the overall distortion is minimised. Accordingly, the transfer characteristic of the predistorter is approximated as closely as possible to the inverse or complementary function of the transfer characteristic of the amplifier. If the linear amplifier is compressive, i.e. the gain tails off at higher power levels, then the predistorter will compensate for this compression by correspondingly expanding the input signal.
Several approaches exist for predistorting the input signal, each differing in the way the predistorter approximates the inverse or complementary function. One approach approximates the inverse function with the exponential characteristics of a diode. One or more diodes may be used together with appropriate biasing to achieve a reduction of the distortion in the order of 10 dB. A second approach is to perform a piece-wise approximation of the inverse function using a series of linear gain, straight line elements interconnected end-to-end. A drawback with this approach is that the alignment and control of the line elements requires complex circuitry owing to the interconnection points having two degrees of freedom.
Polynomial predistortion is another approach to approximating the inverse function of the amplifier transfer characteristic. It is based on a polynomial expansion of the inverse function which may be expressed as follows:
y=a+bx+cx2+dx3+ex4+fx5+gx6+hx7 . . . .
The term a is an offset which may be set to zero in a practical polynomial predistorter. The term bx represents the gain of the predistorter which is linear and merely contributes to the gain of the main amplifier. The terms containing even powers of x represent harmonic distortion components generated in the main amplifier which may be removed using frequency filtering, and therefore these terms may also be set to zero. The remaining terms containing odd powers of x represent in-band distortion caused by the main amplifier (in addition to harmonics which can be filtered as above). In fact, each of these odd-power terms may be considered to represent the equivalent order of intermodulation distortion generated in the main amplifier.
According to a first aspect of the present invention there is provided a predistorter arrangement for linearising an amplifier, the predistorter arrangement comprising an input signal path for receiving an input signal which is required to be amplified, a distortion path in which an input signal from the input signal path is processed to generate a distortion signal comprising at least one distortion component which is a third order or higher order component of the input signal, means for combining the distortion signal with the input signal in the input signal path to produce a predistorted input signal which is supplied to the amplifier input, and an error correction means in which the amplifier output signal is compared with the distortion signal from the distortion path to produce error correction signals for controlling the generation of at least one distortion component in the distortion path, wherein, for each distortion component for which an error correction signal is produced, the error correction means compares the distortion component individually with the amplifier output signal to produce the error correction signal for controlling the generation of that distortion component.
Ideally, the error correction means subtracts from the amplifier output signal the input signal from the input signal path prior to comparing the amplifier output signal with the distortion signal.
In a preferred embodiment, the error correction means supplies the input signal subtracted amplifier output signal to a feed forward arrangement. The application describes the combination of a predistorter arrangement and a feed forward arrangement for linearising an amplifier, in which the input signal subtracted amplifier output signal provides the error signal for the feed forward arrangement.
The distortion path preferably includes means for adjusting the distortion signal in dependence on the error correction signal, and the adjustment means may enable adjustment of the distortion signal in phase and amplitude.
In one embodiment, the adjustment means comprises a variable phase shifter and a variable attenuator.
In another embodiment, the adjustment means comprises an in-phase adjustment means and a quadrature phase adjustment means.
Preferably, the correction means correlates the amplifier output signal with the distortion signal to produce the error correction signal.
According to a second aspect of the present invention there is provided a method for linearising an amplifier, including a distortion step in which an input signal which is required to be amplified is processed to generate a distortion signal comprising at least one distortion component which is a third order or higher component of the input signal, a combining step in which the distortion signal is combined with the input signal to produce a predistorted input signal which is supplied to the amplifier input, and an error correction step in which the amplifier output signal is compared with the distortion signal to produce error correction signals which control the generation of at least one distortion component in the distortion step, wherein for each distortion component for which an error correction signal is produced, the error correction step comprises comparing the distortion component individually with the amplifier output signal to produce an error correction signal for controlling the generation of that distortion component.
This application also describes a predistorter arrangement for linearising an amplifier, the predistorter arrangement comprising an input signal path for receiving an input signal which is required to be amplified, and a distortion path in which an input signal from the input signal path is processed to generate a distortion signal, which is combined with the input signal in the input signal path to produce a predistorted input signal which is supplied to the amplifier input, wherein the distortion path processes the input signal to generate at least two different third order or higher order components of the input signal, and includes means for independently adjusting the phase and amplitude of the at least two components.
From another viewpoint, this application describes a device for predistorting an input signal to an amplifier to compensate for distortion errors generated by the amplifier, the device comprising means for receiving an input signal, a main signal path for supplying the input signal to an input of an amplifier, a distortion signal path having means for generating a distortion signal from the input signal, adjustment means for adjusting the amplitude and phase of the distortion signal, and means for adding the distortion signal to the input signal on the main signal path, wherein the device further comprises error signal generating means for coupling to an output of an amplifier and to the main signal path for generating an error signal to control the adjustment means.
Further features and advantages of the invention will be apparent from the description below.